The present invention relates to a field emission type magnetron used for a high frequency heating device such as a microwave range or a pulse generator such as a radar.
In a usual thermo-electron emission type magnetron, a hot cathode is used as an electron source. The hot cathode supplies electrons by emitting thermo-electrons. A thermo-electron emission is a mechanism that free electrons of a conduction band of the cathode obtain thermal energy by heating a material at about 1500 to 2700 K so that the free electrons get over a surface potential barrier to be emitted to a space.
FIG. 6 is a longitudinally sectional view showing one example of a usual thermo-electron emission type magnetron. In the drawing, a hot cathode 2 is disposed at a central part of a plurality of anode vanes 1. The hot cathode 2 is formed in such a manner that tungsten wire rods 3 including thorium are formed helically at substantially equal intervals and both end parts are held by end hat parts 4. Electric current is supplied to the hot cathode 2 to raise the temperature of the cathode to about 2000K and emit thermo-electrons (for instance, see JP-A-2001-23531).
Further, a field emission type magnetron provided with a cathode that employs a usual field emission phenomenon uses a metallic foil as an electrode, as well known (for instance, see Japanese Patent No.2740793).
The field emission phenomenon means a phenomenon that a high electric field (about 109V/m) is applied to a part near the surface of a material to allow the potential barrier on the surface of the material to be thin and electrons to be emitted outside the material without getting over the potential barrier due to a tunnel effect generated by the surge characteristics of the electrons. In the case of the magnetron of a cm band in which the voltage of the cathode is located within a range of several kV to several ten kV, the field intensity of the surface of the cathode is 107V/m. Thus, when the electric field is not intensified approximately by two digits, the field emission is not generated. Accordingly, in order to realize the field emission in the cathode for the magnetron, an electrode for a field emission needs to have a structure that a radius of curvature of an end is small like a needle or a foil and improve a field concentration effect.
FIG. 7 is a longitudinally sectional view showing the structure of main parts of the usual field emission type magnetron. In the drawing, a cathode part 5 is disposed in a central part of a plurality of anode vanes 1. The cathode part 5 has a structure that a plurality of field emission electrodes 6 made of metallic thin films formed in disk shapes with edges sharpened by an electric corrosive method are combined with a plurality of cathode substrates 8 to which oxide films 7 are applied to have a high secondary electron gain and the combined members are held between end hats 9. The oxide films 7 of the cathode substrates 8 are arranged to emit many secondary electrons when electrons from the field emission electrodes 6 rush.
In the usual thermo-electron emission type magnetron, since the temperature of the cathode reaches about 2000K, an expensive material having a high melting point has needed to be used in the periphery of the cathode.
In the usual thermoelectron emission type magnetron, a heater power source for heating the cathode has been separately necessary as well as a high voltage power source for applying voltage between the anode and the cathode of the magnetron.
The usual thermoelectron emission type magnetron undesirably has a problem that it takes time to supply electric current to the cathode and obtain a desired operating temperature.
Further, the usual thermoelectron emission type magnetron inconveniently needs to raise the temperature of the cathode and the cathode undesirably consumes electric power.
Further, in the usual field emission type magnetron, a work for sharpening the edges of metallic foils as field emission electrodes has been difficult. Further, a stable production has been difficult.
Since field intensities applied to a plurality of field emission electrodes are respectively different, electric currents emitted from the electrodes are not respectively uniform. Therefore, the electrode to which a load is applied is especially seriously exhausted, so that the function of the electrode is firstly deteriorated. This phenomenon undesirably causes the life of the whole of the cathode to be shortened.
Since a large number of field emission electrodes as primary electron emission sources and electrodes as secondary electron emission sources for multiplying electrons emitted from an electric field needs to be coaxially arranged, the number of parts is undesirably large, an assembly thereof is difficult and a cost is high.